Closed loop tracking oscillator

ABSTRACT

A closed loop tracking oscillator is disclosed. A voltage tunable oscillator causes its output frequency to track an input signal by converting a sampled portion of its output frequency to a voltage which varies in accordance with the output frequency. This voltage is compared with the input signal to be tracked to derive an error signal which is amplified and fed to the voltage tunable oscillator to be superimposed upon the input signal to reduce the tracking error toward zero.

United States Patent [111 3,582,311

[72] inventor Floyd E. Kingston 3,221,266 11/1965 Vitkovits, Jr. 33l/4XPalo Alto 3,493,883 2/1970 Mead et al. 331/4 [21] Appl. No. 838,7653,514,712 5/1970 Sinclair etal. 331/178X {5?} 523m iiiiil'l fi jf'r g e-y H G sistant xammerleg ne nmm [73] Asslgnee gram Attorneys-Stanley Z.Cole and Leon F. Herbert [54] CLOSED LOOP TRACKING OSCILLATOR l Claims,3 Drawing Figs.

[521 US. Cl 331/10,

331/178 ABSTRACT: A closed loop tracking oscillator is disclosed. A [51Int. Cl H03b 23/00 voltage tunable oscillator causes its outputfrequency to track [50] Field of Search 331/177, an i t ig al byconverting a sampled portion of its output 177 180v 181,4 10-13frequency to a voltage which varies in accordance with the ututf n.Th'vlta d 't hth 'n t [56] References Cited :ign il to h t rzciid to aeri e a n e r ro iz l g z iiwhi zii is anipiit ltd UNITED STATES PATENTSand fed to the voltage tunable oscillator to be superimposed 3,218,571 1H1965 Shannon etal. 331/4 upon the input signal to reduce the trackingerror toward zero.

AUDIO FREQUENCY -5 FREQUENCY T0 VOLTAGE 1 CONVERTER II FIX ED FREQUENCYM IX ER RAMP VOLTAGE RE OSCILLATOR VOLTAGE TUNABLE V v 5 RE OSCI AT R TLL 0 OUTPUT FVIG.3

PATENTEUJUN H97! 35 2 5311 FIG. i.

5 FREQUENCY T0 VOLTAGE CONVERTER VOLTAGE E OULPUT TUNABLE fi OSCILLATORAUDIO FREQUENCY -:-5 FREQUENCY T0 VOLTAGE Y C(NVERTER FIXED FREQUENCYMIXER RAMPVOLTAGE R REOSCILLATOR E W E VOLTAGE TUNABLE =1", e

o RE (BCILLATOR 1 EACTUALR OUTPUT FREQUENCY DESIRED OUTPUT. FREQUENCYAS'DETERMINED BY. THE

INPUT RAMP VOLTAGE T cKIRG ERROREO Rom E RWRR E BY .J v FREQUENCY E 5 i38 OUTPUT CLOSED LOOP TRACKING OSCILLATOR DESCRIPTION OF THE PRIOR ARTl-Ieretofore, open loop tracking oscillators have been employed toobtain linear sweeps of frequency. Such oscillators have used varactordiodes to provide a voltage variable capacitive element in an L-C tunedcircuit. The varactors were specially selected to match theirvoltage-capacitance curves to the capacity-frequency curves of the tunedcircuit. Such oscillators have provided tuning linearity of fl percentover their entire tuning range at radio frequencies and 10.2 percent ataudiofrequencies.

SUMMARY OF THE PRESENT INVENTION The principal object of the presentinvention is the provision of an improved tracking oscillator.

One feature of the present invention is the provision of afrequency-to-voltage converter for converting a sample of the outputfrequency of a voltage tunable oscillator into a voltage proportional tothe output frequency and comparing the derived voltage with a voltage tobe tracked, in frequency, to derive an error voltage which is amplifiedand fed to the voltage tunable oscillator as a corrective componentsuperimposed upon the tuning voltage to be tracked, thereby minimizingthe tracking error.

Another feature of the present invention is the same as the precedingfeature wherein the voltage tunable oscillator is a radiofrequencyoscillator and a mixer is employed for mixing a stable radiofrequencyreference signal with the output frequency to derive an audio beatfrequency which is proportional to the radiofrequency output frequencyand which beat frequency is converted into the voltage compared with thevoltage to be tracked to derive the error voltage.

Other features and advantages of the present invention will becomeapparent upon a perusal of the following specification taken inconnection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. l is a schematic block diagram ofa closed loop tracking oscillator of the present invention,

FIG. 2 is a schematic diagram of an alternative embodiment of thecircuit of FIG. I, and

FIG. 3 is a plot of voltage versus frequency depicting tracking errorfor oscillators of FIGS. 1 and 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. I, thereis shown a closed loop tracking oscillator l incorporating features 'ofthe present invention. The oscillator ll includes a voltage tunableaudio oscillator 2 having a tracking error, i.e., departure fromlinearity of, for example, :2 percent and which it is desired to reduceby a factor of 10. A sample of the output frequency of the oscillator 2is fed to a frequency-to-voltage converter 3 to produce an outputvoltage proportional to the output frequency of oscillator 2. A typicalfrequency-to-voltage converter 3 includes a differentiator followed by aSchmitt trigger for generating a train of square pulses of uniformheight and width with a frequency proportional to the sampled outputfrequency. The train of square pulses are then integrated by anintegrator to produce the output voltage of an amplitude proportional tothe sampled output frequency.

The output-voltage of the frequency-to-voltage converter 3 is fed to oneinput of an error detector 4 for comparison with an input voltage to betracked as applied to input terminal 5. In a typical example, the inputvoltage comprises a ramp having a precise and constant slope.

The error detector 4 subtracts the two input voltages to derive a DCerror output voltage E, representative of the sense and amplitude of thedeparture of the output frequency from a desired output frequency asdetermined by the input ramp voltage to be tracked. The error voltage E,is fed to an amplifier 6 wherein it is amplified and thence fed to thevoltage tuned input terminal of the voltage tuned oscillator 2 where itis superimposed as a corrective component upon the input ramp voltageapplied directly to the voltage tuned input terminal of the voltagetuned oscillator 2 via lead 7. The inherent tracking error (see FIG. 3)of the voltage tuned oscillator 2, without feedback correction, isdecreased by the factor of the feedback loop gain when feedback isemployed. Thus, if the feedback loop gain is 10 the inherent trackingerror is reduced by a factor of 10. Feedback loop gains of 10 to arereadily achievable to reduce the tracking error to :02 percent to 10.02percent when the oscillator 2 has an inherent tracking error of :2percent.

Referring now to FIG. 2, there is shown an alternative embodiment of thecircuit of FIG. 1. More specifically, the circuit is essentiallyidentical to that of FIG. 1 except that the voltage tunable oscillatoris a radiofrequency voltage tuned oscillator 8, such as a voltage tunedcrystal oscillator operable in the tens to hundreds of MHZ. Theradiofrequency output is sampled and fed to a radiofrequency mixer 9wherein it is heterodyned with a stable fixed radiofrequency derivedfrom a fixed frequency radiofrequency oscillator ll to obtain aconveniently low audiofrequency output. This audiofrequency differs fromthe output radiofrequency by a fixed number of Hz. so that, for example,if the radiofrequency increases by I00 Hz., the audiofrequency alsoincreases by I00 Hz. The audiofrequency output of mixer 9 is then fed tothe frequencyto-voltage converter 3, as in the circuit of FIG. I. Theremainder of the circuit functions in the same manner as thecorresponding portions of the circuit of FIG. I to produce a substantialimprovement in the linearity of the output frequency versus tuningvoltage characteristic.

The advantage of the feedback frequency correction of FIGS. 1 and 2 isthat it permits relatively inexpensive and inherently nonlinear voltagecontrolled oscillators to be employed to provide highly linear tuningvoltage versus output frequency characteristic over relatively widetuning ranges, as of 10 kHz.

Since many changes could be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What we claim is:

1. In a closed loop oscillator circuit, means forming an electricallyvoltage tunable oscillator having an output frequency variable inaccordance with the intensity of a first electrical input voltage tosaid oscillator, means for sampling a frequency detenninative of theoutput frequency of said oscillator and for converting the sampledfrequency to a second electrical voltage having an amplitude whichvaries in accordance with the sampled output frequency of saidoscillator, means forming an error detector, feedback means for feedingsaid second electrical voltage to said error detector for comparisontherein with a third input electrical voltage to be tracked to producean error voltage representative of the frequency deviation of the outputfrequency from a certain desired output frequency as determined by theintensity of said third electrical voltage, means for amplifying theerror voltage and feeding same to said oscillator as a correctivecomponent, and means for feeding said first electrical input voltage asan additional input to said oscillator and said error detector is acomparator circuit for subtracting said second voltage from said thirdvoltage to derive said error voltage.

2. The apparatus of claim 1 wherein said oscillator is a radiofrequencyoscillator, means for heterodyning a sample of the radiofrequency outputsignal from said radiofrequency oscillator with a fixed referenceradiofrequency to derive a difference audiofrequency which isproportional to the output radiofrequency of said oscillator, saidfrequency sampling means being operative upon the audiofrequency toderive said second electrical voltage.

1. In a closed loop oscillator circuit, means forming an electricallyvoltage tunable oscillator having an output frequency variable inaccordance with the intensity of a first electrical input voltage tosaid oscillator, means for sampling a frequency determinative of theoutput frequency of said oscillator and for converting the sampledfrequency to a second electrical voltage having an amplitude whichvaries in accordance with the sampled output frequency of saidosciLlator, means forming an error detector, feedback means for feedingsaid second electrical voltage to said error detector for comparisontherein with a third input electrical voltage to be tracked to producean error voltage representative of the frequency deviation of the outputfrequency from a certain desired output frequency as determined by theintensity of said third electrical voltage, means for amplifying theerror voltage and feeding same to said oscillator as a correctivecomponent, and means for feeding said first electrical input voltage asan additional input to said oscillator and said error detector is acomparator circuit for subtracting said second voltage from said thirdvoltage to derive said error voltage.
 2. The apparatus of claim 1wherein said oscillator is a radiofrequency oscillator, means forheterodyning a sample of the radiofrequency output signal from saidradiofrequency oscillator with a fixed reference radiofrequency toderive a difference audiofrequency which is proportional to the outputradiofrequency of said oscillator, said frequency sampling means beingoperative upon the audiofrequency to derive said second electricalvoltage.